Well, you have to actually hatch all the Zodiacs. They can't be sitting in your storage. And I guess the way you would check for them is to check your album, and/or go to all of your habitats and make a list of all the ones you find in your habitats. it gives you a nice excuse to visit all your animals and really look at them.

Sometimes I get so busy just collecting cash that I forget to really look. Anyway, these are the Zodiac signs - check your album first, probably.
Aries (March 21-April 19)
Taurus (April 20-May 20)
Gemini May 21-June 20)
Cancer (June 21-July 22)
Leo (July 23-August 22)
Virgo (August 23-September 22)
Libra (September 23-October 22)
Scorpio (October 23-November 21)
Sagittarius (November 22-December 21)
Capricorn (December 22-January 19)
Aquarius (January 20 to February 18)
Pisces (February 19 to March 20)